Added option to configure number of worker instances and to set SPARK_MASTER_OPTS
Depends on: https://github.com/mesos/spark-ec2/pull/46
Author: Allan Douglas R. de Oliveira <allan@chaordicsystems.com>
Closes#612 from douglaz/ec2_configurable_workers and squashes the following commits:
d6c5d65 [Allan Douglas R. de Oliveira] Added master opts parameter
6c34671 [Allan Douglas R. de Oliveira] Use number of worker instances as string on template
ba528b9 [Allan Douglas R. de Oliveira] Added SPARK_WORKER_INSTANCES parameter